Create and call python function from string via C

问题:

Is it possible to load a python function from a string and then call that function with arguments and get the return value?

I'm using the python C API to run python code from inside my C++ application. I'm able to load a module from a file using PyImport_Import, get a function object from that using PyObject_GetAttrString, and call the function with PyObject_CallObject. What I'd like to do is to load the module/function from a string instead of a file. Is there some equivalent to PyImport_Import which would allow me to pass it a string instead of a file? I need to pass arguments to the function I'm calling and I need access to the return value, so I can't just use PyRun_SimpleString.

---

Edit:

I found this solution after getting turned on to PyRun_String. I'm creating a new module, getting its dictionary object, passing that along in a call to PyRun_String to define a function in my new module, then getting a function object for that newly created function and calling it via PyObject_CallObject, passing my args. This is what I've found to solve my problem: main.cpp

int main()
{
    PyObject *pName, *pModule, *pArgs, *pValue, *pFunc;
    PyObject *pGlobal = PyDict_New();
    PyObject *pLocal;

    //Create a new module object
    PyObject *pNewMod = PyModule_New("mymod");

    Py_Initialize();
    PyModule_AddStringConstant(pNewMod, "__file__", "");

    //Get the dictionary object from my module so I can pass this to PyRun_String
    pLocal = PyModule_GetDict(pNewMod);

    //Define my function in the newly created module
    pValue = PyRun_String("def blah(x):\n\tprint 5 * x\n\treturn 77\n", Py_file_input, pGlobal, pLocal);
    Py_DECREF(pValue);

    //Get a pointer to the function I just defined
    pFunc = PyObject_GetAttrString(pNewMod, "blah");

    //Build a tuple to hold my arguments (just the number 4 in this case)
    pArgs = PyTuple_New(1);
    pValue = PyInt_FromLong(4);
    PyTuple_SetItem(pArgs, 0, pValue);

    //Call my function, passing it the number four
    pValue = PyObject_CallObject(pFunc, pArgs);
    Py_DECREF(pArgs);
    printf("Returned val: %ld\n", PyInt_AsLong(pValue));
    Py_DECREF(pValue);

    Py_XDECREF(pFunc);
    Py_DECREF(pNewMod);
    Py_Finalize();

    return 0;
}

---

Here is the rest of my original post, left for posterity:

Here's what I was doing originally: main.cpp:

#include <Python.h>

int main()
{
    PyObject *pName, *pModule, *pArgs, *pValue, *pFunc;

    Py_Initialize();
    PyRun_SimpleString("import sys");
    PyRun_SimpleString("sys.path.append('')");
    pName = PyString_FromString("atest");
    pModule = PyImport_Import(pName);
    Py_DECREF(pName);

    if(pModule == NULL)
    {
        printf("PMod is null\n");
        PyErr_Print();
        return 1;
    }

    pFunc = PyObject_GetAttrString(pModule, "doStuff");
    pArgs = PyTuple_New(1);
    pValue = PyInt_FromLong(4);
    PyTuple_SetItem(pArgs, 0, pValue);

    pValue = PyObject_CallObject(pFunc, pArgs);
    Py_DECREF(pArgs);
    printf("Returned val: %ld\n", PyInt_AsLong(pValue));
    Py_DECREF(pValue);

    Py_XDECREF(pFunc);
    Py_DECREF(pModule);

    Py_Finalize();

    return 0;
}

And atest.py:

def doStuff( x):
    print "X is %d\n" % x
    return 2 * x

回答1:

PyRun_String in the Python C API is probably what you're looking for. See: http://docs.python.org/c-api/veryhigh.html

回答2:

The answer contained in the question is excellent but I had some small troubles using it with Python 3.5, so to save anyone else doing what I did, below is a slightly edited version that seems to work fine for this version of Python at least:

#include <Python.h>

int main(void)
{
    PyObject *pArgs, *pValue, *pFunc, *pModule, *pGlobal, *pLocal;

    Py_Initialize();

    pGlobal = PyDict_New();

    //Create a new module object
    pModule = PyModule_New("mymod");
    PyModule_AddStringConstant(pModule, "__file__", "");

    //Get the dictionary object from my module so I can pass this to PyRun_String
    pLocal = PyModule_GetDict(pModule);

    //Define my function in the newly created module
    pValue = PyRun_String("def blah(x):\n\ty = x * 5\n\treturn y\n",
        Py_file_input, pGlobal, pLocal);

    //pValue would be null if the Python syntax is wrong, for example
    if (pValue == NULL) {
        if (PyErr_Occurred()) {
            PyErr_Print();
        }
        return 1;
    }

    //pValue is the result of the executing code, 
    //chuck it away because we've only declared a function
    Py_DECREF(pValue);

    //Get a pointer to the function I just defined
    pFunc = PyObject_GetAttrString(pModule, "blah");

    //Double check we have actually found it and it is callable
    if (!pFunc || !PyCallable_Check(pFunc)) {
        if (PyErr_Occurred()) {
            PyErr_Print();
        }
        fprintf(stderr, "Cannot find function \"blah\"\n");
        return 2;
    }

    //Build a tuple to hold my arguments (just the number 4 in this case)
    pArgs = PyTuple_New(1);
    pValue = PyLong_FromLong(4);
    PyTuple_SetItem(pArgs, 0, pValue);

    //Call my function, passing it the number four
    pValue = PyObject_CallObject(pFunc, pArgs);

    fprintf(stdout, "Returned value: %ld\n", PyLong_AsLong(pValue));

    Py_DECREF(pValue);
    Py_XDECREF(pFunc);

    Py_Finalize();

    return 0;
}